One third of a woman's life is spent in the estrogen-deficient, postmenopausal state. Her lifetime risk of developing cardiovascular disease is 46.1% and of dying of it 31%. For hip fracture due to osteoporosis, the risk of its occurrence is 15.3% and 1.5% for death as a result of it. In people over the age of 55, osteoarthritis is more prevalent and more severe in women. These are all diseases associated with menopause of which cardiovascular disease is one of the main contributors. Sheep have emerged as a convenient large animal model for study of human conditions. They are docile, compliant and are raised as food animals under range conditions. Society does not have the emotional attachment to sheep like the dog. Aged breeding ewes (sheep have no natural menopause) are available in large numbers so that large-scale studies are possible. We have access to large flock (> 10,000) of sheep of known age. Age of the sheep is determined at this ranch by ear tag, a standard marking system. The sheep from this ranch are of one breed (Warhill) and are breeding ewes. There have been very few introductions of other breeds to this ranch since the 1930's and therefore the animals are relatively genetically homogeneous. To obtain aged large animals of any type (primates, dogs pigs) for research into "menopausal diseases" is expensive and more often is impossible. Sheep are skeletally mature (all physes closed) at 3 years. This study will examine the aged ovariectomized (surgical menopause) ewe as a model for cardiovascular disease in postmenopausal women. Skeletally mature "aged" ewes (8 years old), will be ovariectomized (OVX) and some animals will be treated with estrogen. Six months later, histological changes in the coronary and carotid arteries, aorta and iliac arteries will be examined in the sham, OVX and estradiol-treated groups.